I'm a day out from having finished Assassins Creed 2, so the thoughts are still rather fresh in my head.
What they did right:
The game is tall, and open and supports a ton of options for getting from place to place for your missions. They also streamlined alot of the information you recieve this time. In the first game, being removed from the Animus at the end of every mission only to walk around and then get back in was fairly annoying. It's nice that they got rid of it as a result. They also definitely jumpstarted the story a bit more than the first one did. I know this is supposed to be a trilogy of games, but story wise this one was ton's more interesting than the events with Altair, o rperhaps it was just as interesting, only presented way better.
What they can work on:
I love the concept of contextual controls. Playing this game, comes very naturally after your first big fight, but there were a few things i hope they can get a better hang on. Ofter during my 20hours, I kept having the feeling I was playing a Zelda game. The contextual controls there are very similar, only Zelda isn't a frustrating in some of the camer sequences. Jumping is a huge part of this game, but if you're going to have it, then tweak the camera to allow you to jump left or right when you puch that direction. The map needs a bit more detail. Sometimes finding shops listed on the map is a bit of a challenge, as they somehow are around a corner on another street. You should also be able to zoom in a bit more on the map considering how far you can actually zoom out.
Overall, this is a great game to own, and since they created a better UI for accessing the missions/your story/almost everything, it makes me anticipate the next game that much more.